In constructing spatial structures such as steel towers and bridges in the iron & steel industry, a component processing process, a welding process, and a tentative assembly process may be performed in steps from Computer-Aided-Design-based (CAD-based) designing to on-site assembling. Following the tentative assembly process, components are shipped to a construction site so as to perform on-site assembling. In the component processing process, steel components of a spatial structure are processed; in the welding process, steel components are tentatively welded, the tentative welding is diagnosed, and regular welding is performed; in the tentative assembly process, tentative assembling, disassembling, paint finishing, and diagnoses are performed.
Such a spatial structure is often one large item. Accordingly, diagnoses in the welding process and the tentative assembly process are frequently performed through visual checking of comparing a tentatively welded or painted object with a three-dimensional CAD model created in a designing process.
When a fault in the welding process is overlooked in a diagnosis and is found during a tentative assembly task, a returning operation from the tentative assembly process to the component processing process occurs. When a fault in the tentative assembly process is overlooked in a diagnosis and is found during an on-site assembling task, a returning operation from the on-site assembling to the component processing process occurs.
A technique for superimposing and displaying an image of an object and an image of a three-dimensional model is also known in the field of image processing (see, for example, patent documents 1-3).    Patent Document 1: Japanese Laid-open Patent Publication No. 2010-211746    Patent Document 2: Japanese Laid-open Patent Publication No. 11-239989    Patent Document 3: International Publication Pamphlet No. WO 2012/173141